High pressure fluid power systems, such as hydraulic systems, employ gas charged accumulators to serve as energy storage devices and/or auxiliary energy sources. The accumulators are designed to reversibly store a working fluid, such as a hydraulic fluid, from the system under pressure against a trapped cushion of highly compressed gas. For example, in hydraulic brake systems of automobiles, accumulators are employed to store hydraulic energy during low volume demands, smooth pressure spikes, and maintain hydraulic pressure during high volume demands.
Various types of gas charged accumulators are known, such as diaphragm-, bladder-,piston- and bellow-type accumulators. In diaphragm accumulators, in particular, a sealed hollow housing is provided. Positioned within the housing is a sheet-like movable diaphragm composed of a flexible material, such as rubber, to subdivide the interior space into two fluid-tight pressure chambers on opposite sides of the diaphragm. One chamber is a fluid chamber into and out of which the working hydraulic fluid of the hydraulic system is accumulated and expelled under pressure when required. The other chamber is charged with a gas, for example, nitrogen gas, under high pressure to act as an energy storage medium. In use, the working fluid accumulates in the fluid chamber and when the fluid pressure exceeds the pressure of the trapped gas, the diaphragm is elastically displaced, further compressing the gas on the other side. The gas is later utilized to expel on demand the stored fluid under pressure out of the fluid chamber back into the hydraulic fluid system.
In the past, it has been difficult to prevent the diffusion of the charged gas contained within the gas chamber through the diaphragm membrane into the fluid chamber containing the hydraulic fluid. Consequently, standard gas charged accumulators tend to gradually lose their charge and require periodic recharging or replacement. Flexible rubber diaphragms, in particular, are notorious for allowing gas to invariably permeate therethrough and dissolve in the hydraulic fluid, increasing the saturated gas levels in the hydraulic fluid and, thus, rendering the relatively incompressible hydraulic fluid compressible. As one will appreciate, this condition dangerously impairs the performance of the hydraulic system concerned, which, in turn, reduces the reliability of the system and increases service frequencies.
Prior attempts have been made to reduce the gas permeability of rubber diaphragms.
U.S. Pat. No. 5,054,373 (Brault et al.) discloses a gas impermeable, flexible, composite membrane for use in a diaphragm accumulator. The composite membrane comprises three adjoining layers of deformable materials sandwiched together, namely a butyl rubber inner layer, a butyl rubber outer layer, and an intermediate layer nested between the inner and outer layers. The intermediate layer is relatively gas impermeable and comprises a pre-formed metal film, such as aluminum foil. This composite diaphragm, however, suffers from a number of drawbacks. For one, the three separate layers add complexity in construction and undesirable bulk to the accumulator diaphragm. In addition, the lack of adhesion between the layers, which are preferably unbonded, does not allow the desired level of impermeability, rendering the layered composite more susceptible to diffusion and leakage of the charged gas. Furthermore, even when the layers are glued together, the differences in elongation between metal and rubber layers cause relatively large shearing forces to act on the adhered surfaces upon repeated diaphragm displacement, which, in turn, cause delamination and increased gas permeability. The pre-formed metal layer also renders the diaphragm less flexible and durable and more susceptible to destructive deformations. Moreover, there is a tendency for the fatigue-type cracks and fissures induced in the metal layer by flexure to be held permanently open by the opposing fluid pressure acting on the outer rubber layers, even after the diaphragm has returned to a relaxed state. For the foregoing reasons, an accumulator of this kind has only a limited service life. U.S. Pat. No. 5,117,873 (Miyakawa et al.) teaches another sandwiched together rubber-metal foil-rubber composite diaphragm, similar to the one described above, but further showing various ways to reduce gas permeability and leakage through the joint created between the inner wall of the housing and the peripheral edge of the diaphragm.
Recent emphasis on improving the durability of composite diaphragms, especially in cold weather, have resulted in substitutions of the intermediate metal film with more flexible materials made from laminated sheets of gas-impervious synthetic resin films, as shown in U.S. Pat. No. 5,215,124 (Hattori et al.) and U.S. Pat. No. 5,409,041 (Yoshida et al.). U.S. Pat. No. 5,524,671 (Yoshida et al.) discloses another composite membrane for a diaphragm accumulator having sandwiched between the two outer rubber layers, an intermediate layer composed of multiple synthetic resin films laminated together through an adhesive along with an aluminum deposited film interposed between laminated resin layers for enhanced gas barrier performance. Flexible diaphragms made of molded resinous materials, however, tend to be more susceptible to chemical attack by the system fluids, such as brake fluids. Hardening and cracking of the diaphragm upon immersion in the working fluid due in part to plasticizer leaching tends to destroy the gas impermeability of the diaphragm.
What is needed is an accumulator which has a new and improved movable flexible diaphragm that is simple in construction, is essentially impervious to gas diffusion and essentially leak-tight, is not susceptible to chemical attack and deterioration by the system fluids, still maintains high flexibility and volumetric displacement capacity over a wide range of operating temperatures, is able to withstand repeated displacement without creating permanent voids, fatiguing, separating, or delaminating, is reliable and durable and has a long service life, and does not require recharging or frequent servicing.